Generally, with a portable device driven by a battery, final discharge of a battery or battery state up to final discharge is to be detected and indicated, and to do so the battery voltage is detected, converted to digital information by an A/D converter or the like, and periodically monitored. However, by simply detecting the battery terminal voltage a value detected by the A/D converter is lower than the apparent actual terminal voltage due to the occurrence of a voltage drop due to internal resistance of the battery, contact resistance, or resistance of wiring inside the device, that is, a voltage drop to an extent of current consumed by the load. Therefore, in the case of comparing detected terminal voltage with a threshold voltage to determine whether or not the battery has reached a fully discharged state, if the load current is large, in a situation where it should originally have been determined that the battery terminal voltage was still sufficient and that the battery had not reached the fully discharged state, since the apparent terminal voltage is lower, it becomes lower than the threshold voltage and the fully discharged state is erroneously judged.
In the related art, threshold voltage is adjusted using the following methods.    (1) Already-known conditions where consumed electrical current (consumed power) is almost constant are created, and battery terminal voltage in this load state is detected and judgment is performed. With this method, since the load conditions are always the same, it was no longer necessary to correct the threshold voltage. See, for example, Japanese Patent laid-open Publication No. Hei 09-281542.    (2) A small resistance was placed in the power supply line, and a potential difference (that is, consumed current) across both ends of the resistance was detected, and threshold voltage corrected based on this consumed current. With this method, since the consumed current of the battery itself is detected, high precision threshold voltage correction is possible. See, for example, Japanese Patent laid-open Publication No. 2001-021941.In order to ascertain device load conditions, an ON/OFF state of each functional block of a device is managed by a processor, the consumed current (consumed power) at the time of operation of each functional block measured in advance is stored as a table in a memory, and then the consumed current (consumed power) of functional blocks that are in operation at the time of detecting the battery voltage is totaled up, and threshold voltage is corrected using this total value.
Since whether or not the battery has reached the fully discharged state is judged through comparison of detected terminal voltage and a threshold voltage, this is technically equivalent not to correction of threshold voltage, but also to correction of detected terminal voltage.
However, with the above described method (1), occasions arise where conditions in which the consumed current during operation of other functional blocks within the device is not nearly constant, and in this case there is a problem that monitoring of the battery can not be carried out.
Also, with the above described method (2), since a small resistance is inserted in the power supply line and the consumed current detected directly, a need arises to add new devices such as operational amplifiers, and with components having large current consumption there is a new problem that loss is generated in the resistance inserted into the power supply line.
On the other hand, with the above described method (3), these types of problems do not arise, but problems occur depending on the type of display device of the device. That is, in the case of using a liquid crystal display as a display of the device, transmission/shielding is controlled to display shading of an image and so the power consumption is almost constant, but in the case of using a self-emissive display such as an organic EL display, power consumption varies greatly depending on the luminance of the image. Therefore, in a portable device having a self-emissive display, in the case of monitoring the state of a battery using method (3) described above, there is a problem in that it is not possible to correctly monitor battery state using a display image displayed on the self-emissive display. Specifically, in the case of setting a threshold voltage each time a functional block is turned on or off assuming that power consumption of the self-emissive display is a particular fixed value, if the image displayed on the self-emissive display is bright, the apparent terminal voltage is detected as being low, and therefore the threshold voltage become relatively higher than the original threshold voltage. Conversely, if the image displayed on the self-emissive display is dark, the threshold voltage becomes lower than the original threshold voltage, and both of these cases invite erroneous judgment.